1. Field of the Invention
The present invention relates to devices for directly deriving functions of the length of a crack in a test specimen.
2. Prior Art
Testing of various specimens through the use of servo control systems is well known in the art. Examples of such test systems include U.S. Pat. Nos. 3,751,994, 3,546,931 and 3,375,708. In addition, in certain tests a standard test specimen of known dimensions has been loaded cyclically until a crack develops, and then information concerning the material from which the specimen is derived by noting characteristics associated with growth of the crack length in relation to load on the specimen and displacement of the specimen. The American Society for Testing Materials has provided analytical curves for standard specimens defining the relationship between the crack length divided by its width and a compliance factor of the specimen comprising a quantity proportional to the displacement of the specimen divided by the applied load. These curves are obtained from theoretical elasticity solutions or from finite element approximations. It also has been known that these curves may be defined by equations based on the third order of the compliance factor, and thus functions of the crack length have been separately calculated from measurements of displacement and load on the test specimen.
However, many problems have been encountered in attempts to do this on a continuous basis, utilizing electrical signals to provide the necessary output information. One problem has come from the attempt to determine the compliance of the specimen by attempting to measure the peak to peak loads and peak to peak displacements on the specimen during operation. In loading specimens of this type in a cyclic manner, as the specimen starts to crack the displacement will not always return to zero, and inaccuracies will therefore result. Further, a determination of when the peak (maximum) load is reached requires special techniques to assure accuracy.
Thus, while the desirability of providing an electrical output proportional to the desired parameters has long been known and has been recognized by researchers working in the testing field, the means for deriving such an output has escaped people working in the field until the present invention.
The prior art includes cumulative fatigue life indicators as shown in U.S. Pat. No. 3,777,555, and devices to sense upper and lower end limits as shown in U.S. Pat. No. 3,714,821. A test device providing stress-strain read out is disclosed in U.S. Pat. No. 3,826,902. U.S. Pat. No. 3,744,300 discloses apparatus for determining fatigue and random mechanical loads may be applied to a specimen with the circuit disclosed in U.S. Pat. No. 3,597,967. A method of measuring stress by determining displacement is shown in U.S. Pat. No. 3,071,963.
It is believed that none of these patents mentioned as prior art teach ways of providing a direct reading proportional to the length of a crack in a tension specimen.